The present invention relates to a sag compensating roll for a roll press, usable in a paper making machine, or the like. The roll includes a stationary yoke which takes up bending forces, a rotary shell that rotates around the yoke and hydrostatic support elements between the yoke and the shell. Such rolls are known from European Patent EP 0 252 251.
A gap is produced, via the hydrostatic support, between the sliding surface of an individual pressure shoe on the yoke and the inner surface of the roll shell. A certain amount of oil can emerge through that gap, which constantly forms a thin boundary layer of oil. The application force which is applied to the roll shell by the support element varies during the operation of the roll. In that case, the quantity of oil fed to the boundary oil layer also changes. Due to the shear forces in the boundary oil layer between the sliding surface of the pressure shoe and the inner surface of the roll shell, friction is produced upon rotation of the roll shell, which friction heats the oil of the boundary oil layer. Since, as mentioned above, the quantity of oil emerging through the gap varies upon differences in the application force, the temperature in this region also changes. The temperature in the boundary layer decreases with increasing force of application.
Since the application force, of course, differs over the width of the web to be treated, the temperature changes are also different. This has an unfavorable influence on the web. For instance, in the regions of increased application force, a larger quantity of oil is passed through the gap. This results in a stronger cooling action, so that the roll shell contracts more strongly at these places than at other places.
In the embodiment according to FIG. 1 of EP 0 252 251, this problem cannot occur, because in that case, the pressure space 3 is tight. As a result, different quantities of flow cannot result as a function of the linear force. However, the problem can definitely occur when the conditions are different, for instance, in the case of German Patent DE-PS 26 34 821.